Periodic Reporting for period 1 - MONSOON (Monsoons and climate change: roles of atmospheric and oceanic processes)
Reporting period: 2018-11-01 to 2020-10-31
The overall aim of this project is to transform fundamental understanding of monsoon regions in a warming climate. Improving understanding of monsoons is a key science goal with societal implications: Monsoon regions are home to approximately half the global population, and the rainfall delivered by monsoons is vital for communities in Asia, Africa, Australia and the Americas. However, climate-model projections for how monsoon regions will change by the end of the 21st century are highly uncertain. This uncertainty makes it difficult for vulnerable regions to prepare for the impacts of climate change and highlights the lack of a robust understanding of the physical processes driving changes in monsoonal climates.
Existing research has focused primarily on the roles of land processes, aerosols and natural variability (e.g. El Niño-Southern Oscillation) in shaping monsoons. However, the roles of radiative and oceanic processes in driving monsoonal climates are less well understood, and represent potential sources of uncertainty in future projections. To address this knowledge gap, in this project we used a range of climate simulations together with theory & observational data to advance understanding of how clouds, water vapour, carbon dioxide and ocean temperatures shape monsoon regions. The project was structured around two Work Packages (WPs) which address the following questions:
WP1: How do the radiative effects of clouds, water vapour and carbon dioxide couple to monsoon circulations and shape their response to climate change?
WP2: What role do ocean surface temperatures play in driving extreme temperatures in monsoon regions and across tropical continents?
Below we describe the work performed to address each question, the main results achieved and the potential impacts.
Existing research has focused primarily on the roles of land processes, aerosols and natural variability (e.g. El Niño-Southern Oscillation) in shaping monsoons. However, the roles of radiative and oceanic processes in driving monsoonal climates are less well understood, and represent potential sources of uncertainty in future projections. To address this knowledge gap, in this project we used a range of climate simulations together with theory & observational data to advance understanding of how clouds, water vapour, carbon dioxide and ocean temperatures shape monsoon regions. The project was structured around two Work Packages (WPs) which address the following questions:
WP1: How do the radiative effects of clouds, water vapour and carbon dioxide couple to monsoon circulations and shape their response to climate change?
WP2: What role do ocean surface temperatures play in driving extreme temperatures in monsoon regions and across tropical continents?
Below we describe the work performed to address each question, the main results achieved and the potential impacts.
WP1: Radiative effects of clouds, water vapour and carbon dioxide on monsoons
Clouds, water vapour (WV) and carbon dioxide are abundant in the atmosphere above monsoon regions. However, the influences of these processes – and in particular their radiative effects – on monsoon circulations have not been systematically investigated. In WP1, we used a novel suite of idealised climate-model simulations to quantify the individual roles of clouds, WV and carbon dioxide in controlling both the present-day monsoon and changes in the monsoon in response to an increase in carbon dioxide concentration. For the present-day monsoon, we found that seasonal north/south migrations of clouds and WV are fundamental for shaping the monsoon; in the absence of these seasonal feedbacks, monsoon rainfall would be substantially stronger and the timing of monsoon onset would shift by several weeks. Under an abrupt increase in carbon dioxide concentration, changes in clouds exert a strong control on the monsoon response. Changes in WV and carbon dioxide also influence the monsoon response, and their effects are comparable both in magnitude and spatial structure. These results have been published in Byrne & Zanna (2020) and featured in a review article on monsoons & climate change written on behalf of the UN’s World Meteorological Organisation and co-authored by the researcher (Wang, Biasutti, Byrne et al., 2021).
Following on from the work described above, we are also using observational datasets to quantify the coupling between clouds, circulation and top-of-atmosphere energy fluxes both in monsoon regions and across the tropics. We have found that the atmospheric circulation is playing a central role in controlling clouds and their radiative effects on both year-to-year and decadal timescales. This is an important new insight into how tropical clouds affect Earth’s energy balance, with important implications for estimating how climate will change over the coming decades. This work is expected to be submitted for publication in early 2023.
WP2: Influence of the ocean on extreme temperatures in monsoon regions and across tropical continents
Extreme temperatures are becoming more frequent and more intense as climate warms, affecting humans and ecosystems around the world. However, despite this societal importance, the physical drivers of extreme temperatures in a changing climate are not well understood, particularly in tropical regions. In WP2, we investigated the hypothesis that – because of active convection and weak Coriolis forces in the tropical atmosphere – ocean temperatures impose a strong constraint on extreme temperatures over tropical continents. We developed this hypothesis into a novel quantitative theory for the tropical land temperature distribution in a changing climate. This theory – now known as the ‘drier get hotter’ theory – links extreme temperatures over monsoonal regions and tropical land to the surface temperature of neighbouring oceans and to the dryness of the land surface. The theory has been verified using state-of-the-art climate simulations, demonstrating – for the first time – that tropical heatwaves are tightly controlled by the oceans. This work was recently published (Byrne, Nature Geoscience, 2021) and has established a new framework of understanding for extreme temperatures that is expected to shape the next decade of tropical heatwave research.
Overview of the dissemination and exploitation of the project results:
The project results have been widely disseminated to the international climate-science community, including 6 conference talks, 6 workshop talks and 11 invited seminars over the course of the fellowship. Project results have also been disseminated through four scientific publications in leading journals, with a further publication expected in 2023. In addition, a variety of activities have been undertaken during the project to communicate with broader society, including:
• Article aimed at a broad audience in Horizon (the EU research magazine) showcasing the project research.
• Talks communicating the latest climate science for the general public, including at the Royal Society of Edinburgh in 2021.
• The researcher has an established following on Twitter (~1,700 followers) and uses this platform to communicate project activities. For example, a tweet highlighting the Byrne (Nature Geoscience, 2021) study generated more than 2,000 engagements.
Clouds, water vapour (WV) and carbon dioxide are abundant in the atmosphere above monsoon regions. However, the influences of these processes – and in particular their radiative effects – on monsoon circulations have not been systematically investigated. In WP1, we used a novel suite of idealised climate-model simulations to quantify the individual roles of clouds, WV and carbon dioxide in controlling both the present-day monsoon and changes in the monsoon in response to an increase in carbon dioxide concentration. For the present-day monsoon, we found that seasonal north/south migrations of clouds and WV are fundamental for shaping the monsoon; in the absence of these seasonal feedbacks, monsoon rainfall would be substantially stronger and the timing of monsoon onset would shift by several weeks. Under an abrupt increase in carbon dioxide concentration, changes in clouds exert a strong control on the monsoon response. Changes in WV and carbon dioxide also influence the monsoon response, and their effects are comparable both in magnitude and spatial structure. These results have been published in Byrne & Zanna (2020) and featured in a review article on monsoons & climate change written on behalf of the UN’s World Meteorological Organisation and co-authored by the researcher (Wang, Biasutti, Byrne et al., 2021).
Following on from the work described above, we are also using observational datasets to quantify the coupling between clouds, circulation and top-of-atmosphere energy fluxes both in monsoon regions and across the tropics. We have found that the atmospheric circulation is playing a central role in controlling clouds and their radiative effects on both year-to-year and decadal timescales. This is an important new insight into how tropical clouds affect Earth’s energy balance, with important implications for estimating how climate will change over the coming decades. This work is expected to be submitted for publication in early 2023.
WP2: Influence of the ocean on extreme temperatures in monsoon regions and across tropical continents
Extreme temperatures are becoming more frequent and more intense as climate warms, affecting humans and ecosystems around the world. However, despite this societal importance, the physical drivers of extreme temperatures in a changing climate are not well understood, particularly in tropical regions. In WP2, we investigated the hypothesis that – because of active convection and weak Coriolis forces in the tropical atmosphere – ocean temperatures impose a strong constraint on extreme temperatures over tropical continents. We developed this hypothesis into a novel quantitative theory for the tropical land temperature distribution in a changing climate. This theory – now known as the ‘drier get hotter’ theory – links extreme temperatures over monsoonal regions and tropical land to the surface temperature of neighbouring oceans and to the dryness of the land surface. The theory has been verified using state-of-the-art climate simulations, demonstrating – for the first time – that tropical heatwaves are tightly controlled by the oceans. This work was recently published (Byrne, Nature Geoscience, 2021) and has established a new framework of understanding for extreme temperatures that is expected to shape the next decade of tropical heatwave research.
Overview of the dissemination and exploitation of the project results:
The project results have been widely disseminated to the international climate-science community, including 6 conference talks, 6 workshop talks and 11 invited seminars over the course of the fellowship. Project results have also been disseminated through four scientific publications in leading journals, with a further publication expected in 2023. In addition, a variety of activities have been undertaken during the project to communicate with broader society, including:
• Article aimed at a broad audience in Horizon (the EU research magazine) showcasing the project research.
• Talks communicating the latest climate science for the general public, including at the Royal Society of Edinburgh in 2021.
• The researcher has an established following on Twitter (~1,700 followers) and uses this platform to communicate project activities. For example, a tweet highlighting the Byrne (Nature Geoscience, 2021) study generated more than 2,000 engagements.
This project has substantially advanced understanding of climate dynamics beyond the state of the art. In Work Package 1, we have demonstrated for the first time the central importance of radiative processes associated with clouds, WV and carbon dioxide in controlling monsoon circulations in both current and future climates. This research has not only advanced fundamental monsoon science but will also have potentially large socio-economic impacts, as it suggests that persistent uncertainties in monsoon projections are partially driven by cloud and water vapour feedbacks, opening up new pathways to more reliable monsoon forecasts.
In Work Package 2, we have developed & tested the first quantitative theory to understand how extreme tropical temperatures respond to climate change (Byrne, Nature Geoscience, 2021). This research constitutes a step-change in our scientific knowledge of tropical heatwaves, a severe emerging threat with widespread societal implications. This project has also helped to establish extreme temperatures as a new area of focus for the researcher and his group, and has stimulated follow-up funding applications (e.g. a pending ERC Starting Grant submitted in January 2022).
In Work Package 2, we have developed & tested the first quantitative theory to understand how extreme tropical temperatures respond to climate change (Byrne, Nature Geoscience, 2021). This research constitutes a step-change in our scientific knowledge of tropical heatwaves, a severe emerging threat with widespread societal implications. This project has also helped to establish extreme temperatures as a new area of focus for the researcher and his group, and has stimulated follow-up funding applications (e.g. a pending ERC Starting Grant submitted in January 2022).